1. Field of the Invention
This invention relates generally to the evaluation of a wafer of semiconductor material, and in particular to the measurement of a property of the semiconductor material.
2. Description of Related Art
In the processing of a semiconductor wafer to form integrated circuits, charged atoms or molecules are directly introduced into the wafer in a process called ion implantation. Ion implantation normally causes damage to the lattice structure of the wafer, and to remove the damage, the wafer is normally annealed at an elevated temperature, typically 600.degree. C. to 1100.degree. C. Prior to annealing, material properties at the surface of the wafer may be measured, specifically by using the damage caused by ion implantation.
For example, U.S. Pat. No. 4,579,463 granted to Rosencwaig et al. (that is incorporated herein by reference in its entirety) describes a method for measuring a change in reflectance caused by a periodic change in temperature of a wafer's surface (see column 1, lines 7-16). Specifically, the method uses "thermal waves [that] are created by generating a periodic localized heating at a spot on the surface of a sample" (column 3, lines 54-56) with "a radiation probe beam . . . directed on a portion of the periodically heated area on the sample surface," and the method "measur[es] the intensity variations of the reflected radiation probe beam resulting from the periodic heating" (column 3, lines 52-66).
As another example, U.S. Pat. No. 4,854,710 to Opsal et al. (also incorporated herein by reference in its entirety) describes a method wherein "the density variations of a diffusing electron-hole plasma are monitored to yield information about features in a semiconductor" (column 1, lines 61-63). Specifically, Opsal et al. state that "changes in the index of refraction, due to the variations in plasma density, can be detected by reflecting a probe beam off the surface of the sample within the area which has been excited" (column 2, lines 23-31) as described in "Picosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium," by D. H. Auston et al., Physical Review Letters, Vol. 32, No. 20, May 20, 1974.
Opsal et al. further state (in column 5, lines 25-31 of U.S. Pat. No. 4,854,710): "The radiation probe will undergo changes in both intensity and phase. In the preferred embodiment, the changes in intensity, caused by changes in reflectivity of the sample, are monitored using a photodetector. It is possible to detect changes in phase through interferometric techniques or by monitoring the periodic angular deflections of the probe beam."
A brochure entitled "TP-500: The next generation ion implant monitor" dated April, 1996 published by Therma-Wave, Inc., 1250 Reliance Way, Fremont, Calif. 94539, describes a measurement device TP-500 that requires "no post-implant processing" (column 1, lines 6-7, page 2) and that "measures lattice damage" (column 2, line 32, page 2). The TP-500 includes "[t]wo low-power lasers [that] provide a modulated reflectance signal that measures the subsurface damage to the silicon lattice created by implantation. As the dose increases, so does the damage and the strength of the TW signal. This non-contact technique has no harmful effect on production wafers" (columns 1 and 2 on page 2). According to the brochure, TP-500 can also be used after annealing, specifically to "optimize . . . system for annealing uniformity and assure good repeatability" (see bottom of column 2, on page 4).